[WebAssembly] Backport custom import name changes for clang to 8.0.

Specifically, this backports r352106, r352108, r352930, and r352936
to the 8.0 branch. The trunk patches don't apply cleanly to 8.0 due to
some contemporaneous mass-rename and mass-clang-tidy patches, so
this merges them to simplify rebasing.

r352106 [WebAssembly] Add an import_module function attribute
r352108 [WebAssembly] Add WebAssemblyImportModule to pragma-attribute-supported-attributes-list.test
r352930 [WebAssembly] Add an import_field function attribute
r352936 [WebAssembly] Fix ImportName's position in this test.

By Dan Gohman!

llvm-svn: 353834